This study demonstrates that seizures in unrestrained, spontaneously breathing animals produce very high elevations of osmolarity in blood plasma, for which release of lactic acid produced by muscular contractions appears to be mainly responsible. The high plasma osmolarity induces osmotic dehydration of the brain which last for several hours. Elevation of plasma osmolarity and dehydration of the brain are absent in animals in which muscular contractions were pharmacologically abolished and which were artificially ventilated. In both groups of animals, it was evident that the opening of the blood-brain barrier (BBB) to protein tracers induces an increase in water contents, which, eventually, may lead to development of edema.